Described herein are solid ink compositions used in printing masks for printed circuit boards.
Printed circuit boards (PCBs) or printed wiring boards (PWBs) (hereinafter collectively PCB's) are platforms that connect and interface electronic components with each other and with other elements in computers, communication devices, consumer electronics, automated manufacturing and inspection equipment. PCB's may be produced from a base of insulating material on which a thin copper layer is laminated or plated, known as a bare copper plated board, from which a chemical etching step selectively removes areas of the copper to produce electrically conducting paths or traces. The traces permit electrical interconnection of the components attached to the PCB.
An insulative material, referred to as a solder mask, is then applied over the copper conducting paths. Solder masks protect the conducting paths on the PCB from being coated with solder during the soldering step, while leaving uncovered only the conducting pads that need to be covered with molten solder. The solder mask layer on simple PCBs may be produced using screen-printing or spin-casting techniques. However, more densely populated PCBs, having generally complicated multi-layer conducting paths, typically utilize photolithography to form a patterned solder mask on the copper layer. Without the photolithographically patterned solder mask, other areas of the conducting paths could also be covered with solder and cause several problems such as short circuits by bridging solder.
During the final lithography step, an additional masking layer or “printed mask” is printed on top of the solder mask, such as those described in U.S. Pat. No. 6,872,320 and U.S. Patent Application Pub. Nos. 2005/0164121, 2008/0241712, 2009/0123873, each of which is incorporated by reference herein in their entirety. The pattern mask may be referred to as a UV mask if the pattern mask functions to block UV light from reaching the solder mask. The PCB is then exposed to UV radiation to crosslink or harden the portion(s) of the solder mask unprotected by the printed mask (the “exposed portion”).
Upon exposure, the printed mask forms an “unexposed portion” of the solder mask by blocking out sufficient UV radiation from reaching the photosensitive material and thus preventing its chemical alteration. For example, UV radiation may be absorbed or reflected by one or more of UV radiation blocking agents in the composition. Subsequent to exposure, the PCB is then exposed to an aqueous base solvent, which strips or washes away both the printed mask and the “unexposed portion” of the solder mask.
While the aqueous base solvent removes some of the unexposed solder mask, such removal is often inadequate or inefficient because these solvents do not strip off solder mask and the printed mask quickly or in sufficient detail as to not require further removal effort, such as undercutting.